All DC power supplies produce noise at their output. The noise can be random, periodic or a combination of the two. In a phase-controlled power supply, the dominant noise is periodic and a multiple of the line frequency; and in a switching supply, one grouping of noise terms is the switching frequency or twice that frequency. In both designs, noise is attenuated by filters, usually of the LC type, in which the components tend to be large and expensive. In power supplies regulated by varying the conductance of a series pass transistor, most of the noise is eliminated by the rheostat action, but the losses in the transistor significantly reduce efficiency. An increase in efficiency and good noise rejection have been achieved by a hybrid supply in which a phase control or switcher is used to maintain a constant low voltage across a series pass transistor that is in the control loop of the hybrid supply; but because the series pass transistor is operated in a linear mode, it still dissipates considerable energy.